While only the keywords listed in 3.1.1.1
and 3.1.1.2 are formally reserved by the
rules of FITS, standard meanings for others may be adopted by general agreement.
Section 4 discusses a number of such keywords.
Conventions for keyword meanings can also be developed within a given discipline,
as has been done for high energy astrophysics, radio interferometry, and single
dish radio observations. Some of these conventions are discussed in section
5. Keywords should not be used in a way
that conflicts with a generally accepted convention. If the convention applies
to all FITS files, avoid using the keyword with a meaning other than that of
the convention; if the convention is discipline-specific, say, to high energy
astrophysics or radio astronomy, a different meaning may apply to a different
discipline. Keyword meanings in conflict with widely accepted usage may confuse
readers.

For keywords that have no value or are used to transfer text, such as
HISTORY, COMMENT, and REFERENCE, the same keyword may
be used repeatedly to extend text over a sequence of card images.
However, repetition of a keyword with conflicting values may cause
confusion. There is no standard interpretation in such a case
determining which value is to be retained as the true value of the
keyword. Different FITS readers may interpret the header differently. Do
not repeat a keyword on different card images if the values conflict.

The comment field following the keyword and value provides an
opportunity to explain the meanings. Such comments should supply
additional information. Some FITS writers automatically generate
comments such as ``Physical coordinate of axis'' following a
CTYPEn keyword or ``Physical units of matrix'' following a
BUNIT keyword. Such comments provide no more information than has
been given by the FITS rules and are not very helpful for the human
reader. Comments should provide information not available from the FITS
syntax, for example, what the axis is (right ascension? declination?
frequency?) or the units of the values in the primary data array
(Janskys/beam? magnitudes/arcsec?). The number of characters
in a string value is often limited to accommodate reading software,
resulting in abbreviations; the comment field can and should be used to
explain these abbreviations in full. While the meaning of an
abbreviation may be obvious at the writing installation, it may not be
so clear to a reader.